The use of metal coatings for decoration and ornamentation began several thousand years ago. However, only within the last hundred years have metallic pigments become commercially important. Historically, the value of surfaces covered with gold or other metals resided not only in an aesthetically pleasing, bright, metallic finish, but in the ability of such surface coatings to withstand the ravages of time and weathering of the elements better than any other available type of surface coating. The high cost of gold or other metals, made it become difficult to produce a suitable thin leaf and the use of metallic coatings was limited to jewelry, porcelain, chinaware and other art objects. To produce thin leaf or coating of metal that was only a few thousandths of an inch thick, it was necessary to begin with a ductile metal that was already hammered into extremely thin sheets. These sheets were then interleaved with animal skins and further hammered until the resultant foil was fine enough to be used. During this process the edges of the thin leaf broke off into small particles. It was then found that by mixing these small fine flakes with a drying oil, a finish could be obtained that came close to resembling a continuous sheet of metal. The artisans who worked with this type of finish on a large scale prepared their metallic pigments by rubbing the finely hammered metal through a fine metal mesh.
During the mid 1800's, Bessemer produced the first practical and economical method to manufacture metallic flake pigments. This was accomplished by stamping or hammering metal sheets of appropriate brightness and then reducing the sheets into flakes which were graded and collected.
Charles Hall and Paul Herroult independently invented a practical aluminum smelting process in 1886 causing aluminum to be available in commercial quantities. Aluminum was technically adaptable to the Bessemer process, but the drawback was that it formed an explosive mixture with air over a wide range of metal-air ratios.
In 1925, Everett Hall was granted a number of patents for producing a safe and superior aluminum flake pigment. This Hall process, which employed a wet ball mill, carried out the size reduction of aluminum in the presence of a paint thinner containing a lubricant in solution. The lubricant was used to prevent heat cohesion of the fine flakes and the choice of lubricants determined the type of flake formed. In this process, the explosive potential from the finely powdered aluminum was minimized and a large scale commercial manufacturing process was developed. An example of the results of this invention was the paint used in 1931 to cover all structural parts of New York's George Washington Bridge.
In modern times, metallic coatings are obtained by using conventional aluminum flake and powder pigments which are formed as inks and then applied by printing methods. The metallic pigments are obtained by condensation of metallic vapors, electroplating, direct vacuum sputtering or transformed from foil leaf. The coatings using conventional aluminum pigments are gray, or, at best, are very low reflective coatings. The coatings are typically expensive, the processes difficult to control, and the processes do not lend themselves to high volume continuous coating applications. Examples of metallic coating compositions and processes for making metallic pigments are disclosed in U.S. Pat. No. 2,941,894 to McAdow, U.S. Pat. No. 2,839,378 also to McAdow, and U.S. Pat. No. 4,116,710 to Heikel.
Diagrams illustrating typical aluminum pigment production is described in FIG. 16 on p. 799 of Pigment Handbook, Volume 1, of J. Wiley & Sons, New York and FIG. 5 on p. 5 of Section FA2C-1, Powder and Pigments, July 1976, Alcoa Aluminum Pigments Products Data.
Aluminum pigments, prepared as described above, have been used in paints, enamels, lacquers and other coating compositions and techniques. The various grades of fineness of conventional aluminum pigments vary from relatively coarse particle sizes such as 250 microns (50 mesh) to about 44 microns (325 mesh).
A drawback of conventional aluminum and metallic pigments is their nugget-like shape. In formulating compositions containing the conventional shaped aluminum pigments, different particle sizes, concentrations as high as 30% by weight are usual. Due to the shape of the aluminum pigments particles, the particles tend to protrude from the surface of the ink or paint vehicle after drying, causing a phenomenon called "dusting" which occurs when the dried coating is rubbed, thereby removing some of the metallic residue. In addition, because the pigment particles do not lie flat and are randomly distributed, the plate out is usually not uniform and requires multiple coats or applications. An additional drawback is the milling entailed in the process of size reduction in which the original brightness of the metal deteriorates and the metal takes on a gray appearance.
Many of these shortcomings in traditional processes were significantly resolved by the process described in U.S. Pat. No. 4,321,087 to Sol Levine et al. The Levine et al. process produces very thin, bright metallic flakes with extremely smooth (mirror-like) surfaces. The flakes serve as excellent pigments and, when properly employed can offer metal-like or mirror-like optical effects.
In a parallel development, diffraction patterns and embossments, and the related field of holographs, have begun to find wide-ranging practical applications due to their aesthetic and utilitarian visual effects. One very desirable decorative effect is the iridescent visual effect created by a diffraction grating. This striking visual effect, attributed to Sir John Barton, Director of the British Royal Mint (circa 1770), occurs when ambient light is diffracted into its color components by reflection from a diffraction grating. A diffraction grating is formed when closely and regularly spaced grooves (5,000 to 11,000 grooves per cm.) are embossed on a reflective surface.
In recent times, this diffraction grating technology has been employed in the formation of two-dimensional holographic images which create the illusion of a three-dimensional image to an observer. This holographic image technology can form very attractive displays. Furthermore, because the economics of forming holographic images is significantly dependent upon economies of scale, the concept of using holographic images to discourage counterfeiting has found wide application.
The original diffraction gratings were formed by scribing closely and uniformly spaced lines on polished metal surfaces using special "ruling engines." Subsequently, techniques were developed to reproduce a master diffraction grating by shaping a moldable material against the master diffraction grating surface. More recently, thermoplastic films have been embossed by heat softening the surface of the film and then passing them through embossing rollers which impart the diffraction grating or holographic image onto the softened surface. In this way, sheets of effectively unlimited length can be decorated with the diffraction grating or holographic image on a surface. The decorated surface of polymers is sometimes sufficiently reflective that the optical effect of the diffraction grating occurs without further processing, because the incident light is reflected by the facets of the decorated surface. Generally, however, the full optical effects require metallizing of the polymer surface. For the purpose of this application, the term diffraction grating includes holographic images that are based on diffraction grating technology.
It is the general object of the present invention to provide a process for making very thin, bright embossed metallic flake pigments rapidly and inexpensively.
Another object of the present invention is to provide metallic flakes embossed with a machine readable image, such as a conventional bar code image or a holographic bar code image.
Another object of this invention is to provide a process for continuously making embossed metallic flake pigments rapidly and inexpensively.
Another object of the present invention is to provide thin, bright, embossed metallic pigments.
Still another object of the present invention is to provide coating and printing formulations containing the thin, bright embossed metallic pigments of this invention.
Another object of the present invention is to provide an embossed organic or metallic flake useful for security applications.
These and other objects, features and advantages of the present invention will become evident from the following detailed description of the invention taken in conjunction with the drawings.